Dr Paul Lee, Professor John Eisman and Associate Professor Jackie Center, researchers at Sydney's Garvan Institute of Medical Research, examined a cohort of 42 Intensive Care Unit (ICU) patients. Forty-five percent turned out to be Vitamin D deficient. ... When the team correlated the Vitamin D levels with a disease severity score, there was a direct correspondence between sickness and Vitamin D deficiency. In other words, the sicker someone was, the lower the levels of Vitamin D. Out of the 42 patients studied, there were 3 deaths. The 3 patients who died all had the lowest level of Vitamin D in the cohort.
The big glaring issue here is causality. Is the level of Vitamin D just an indicator of someone who is ill, or a reason they are prone to being ill? More research needs to be done:
The next step will be a randomised control study to investigate whether Vitamin D has benefits in critically ill patients. In simple terms, two groups of patients (who are evenly matched) will be treated, with Vitamin D added to the treatment of one group, but not the other. The outcomes will then be compared. So should doctors be trying to raise the Vitamin D levels of their patients in the meantime? Dr Lee hopes the randomised study may provide a more definitive answer to the question. "However, Vitamin D is very safe. It's inexpensive and has a very large safety window, making toxicity unlikely, unless there are underlying diseases causing high calcium. Giving vitamin D to severely deficient patients is very unlikely to cause harm. In addition, ICU patients are lying in bed for a long time, and are at risk of bone loss and osteoporosis. So if nothing else, Vitamin D will help protect their bones."
Supplementation may not be a cure all, and we may find that all these correlations are popping out of the fact that Vitamin D levels are a proxy for suppressed immune systems in some manner. But the downsides of supplements are probably minimal. Here's the letter in The New England Journal of Medicine.